;(function(designWidth, maxWidth) {
    var doc = document,
    win = window,
    docEl = doc.documentElement,
    remStyle = document.createElement("style"),
    tid;

    function refreshRem() {
      var width = docEl.getBoundingClientRect().width;
      maxWidth = maxWidth || 540;
      width>maxWidth && (width=maxWidth);
      var rem = width * 200 / designWidth;
      remStyle.innerHTML = 'html{font-size:' + rem + 'px;}';
    }

    if (docEl.firstElementChild) {
      docEl.firstElementChild.appendChild(remStyle);
    } else {
      var wrap = doc.createElement("div");
      wrap.appendChild(remStyle);
      doc.write(wrap.innerHTML);
      wrap = null;
    }
    //要等 wiewport 设置好后才能执行 refreshRem，不然 refreshRem 会执行2次；
    refreshRem();

    win.addEventListener("resize", function() {
      clearTimeout(tid); //防止执行两次
      tid = setTimeout(refreshRem, 300);
    }, false);

    win.addEventListener("pageshow", function(e) {
      if (e.persisted) { // 浏览器后退的时候重新计算
        clearTimeout(tid);
        tid = setTimeout(refreshRem, 300);
      }
    }, false);

    if (doc.readyState === "complete") {
      doc.body.style.fontSize = "16px";
    } else {
      doc.addEventListener("DOMContentLoaded", function(e) {
        doc.body.style.fontSize = "16px";
      }, false);
    }
  })(750, 750);